Modify RosRobotModel to derive from sot::Dynamic

The RosRobotModel class is deriving from dynamicgraph::Entity and implements signals and methods from sot::Dynamic.
The modified RosRobotModel derives directly from sot::Dynamic to allow its use as dynamic entity of a robot

WARNING : This class depends on the modified jrl_dynamics_urdf from https://github.com/laas/jrl_dynamics_urdf/pull/6